There is a separation membrane, made of a specific polymer compound, which performs in selective permeation and separation of a desired gas component. As a specific industrial application, the membrane has been utilized in a separation process for eliminating carbon dioxide or the like from a natural gas or bio gas mainly formed of methane and carbon dioxide in the mixture (such gases are generated by fermentation and anaerobic digestion of excreta of organisms, organic fertilizers, biodegradable substances, polluted water, garbages, energy crops or the like). Besides, study has been conducted, in relation to a global warming issue, for separating and recovering carbon dioxide from a large-scale carbon dioxide source such as in a thermal power station, a cement plant, a blast furnace in a steel plant. As such, the membrane separation technique attracts attention as a solution to the above environmental issues, achievable with relatively small energy (see Patent Literature 1, Patent Literature 2 or the like).
Various types of the gas separation membrane have been proposed. One of which includes a separation composite membrane formed of a separation membrane and a porous support in combination. Patent Literature 1 discloses a separation composite membrane containing cross-linked polymer, employed with a porous membrane having a cutoff molecular weight of 500,000 or less (mean pore diameter: 50 nm or less). As disclosed, the separation membrane may perform excellently in bending properties and diminishing pinholes. Patent Literature 2 discloses a separation membrane formed in the manner that a porous membrane having a pore diameter as large as about 1 μm is impregnated into a mixed liquid of a siloxane compound. The art may purportedly achieve a high separation factor and a high permeability coefficient. Further, Patent Literature 3 discloses a separation membrane formed of a fluorocarbon resin provided onto a biaxially oriented polyethylene film. In the literature, this fluorocarbon resin separation membrane is described to preferably exist wholly over a surface and an inside of a porous layer. Consequently, it is concluded that the membrane may attain excellent gas permeation and gas separation properties in separation of oxygen and nitrogen.